When we think about lightning, it is often in the dramatic context of skyscrapers being struck or, for those of you with a nervous disposition, walking across open land such as a golf course, in a thunderstorm.
Of more relevance to Lux readers is the risk to streetlights and floodlights mounted on columns and high masts. In contrast to HID lamps, LEDs are only designed for low operating voltages and they are therefore more sensitive to overvoltage. This equally applies to the drivers, especially in comparison with old fashioned wire-wound ballasts. Some drivers will only withstand a 2kV transient overvoltage.
It should be remembered that there are two basic types of supply. In urban areas, the streetlights are most likely to be connected to the underground mains supply; this supply also feeds the housing and business premises such as shops and offices. As such, energy can be dissipated over a large network.
The other method is to have a dedicated mains supply purely for the streetlighting. Here, the electricity supply company provides a mains cable to a single distribution point; often a feeder pillar or main distribution cabinet. This is known in the UK as a private supply or private network. It is often used on traffic routes where there is no other development or the floodlighting of sports pitches where individual switching or metering is required.
How damage can occur:
There are several ways that an overvoltage can occur and possibly damage the luminaires.
2/ Far less common in terms of damage, surges can also occur on the power grid from switching of heavy electrical loads nearby or the sudden interruption of the supply.
It is sometimes thought that circuit breakers and fuses can provide overvoltage protection, but they are designed to protect equipment, from an event such as a short circuit or overload.
In general electrical installations in buildings, Surge Protective Devices, SPDs, are used to protect equipment from events such as extremely short duration high voltage spikes. These voltage spikes or transients can be caused by anything from switching on a lamp to a lightning storm. Most spikes are of low energy. However, some spikes could possibly cause damage if no SPD is installed to redirect the harmful voltage away from the equipment.
Whilst SPDs are common in buildings, they are much less used in exterior applications. This can put streetlights and floodlights at risk.
As a consequence, in order to give extra lightning protection to their equipment, Osram have fitted a range of their Optotronic drivers with extra overvoltage protection.
Drivers in the Optotronic 2DIM, 3DIM, and 4DIM series have an integrated efficient overvoltage protection which can, depending on the product family, absorb overvoltages up to 8kV. This applies to the mains (phase and neutral conductor) and ground. Between phase (L) and the neutral conductor (N), up to 6kV can be absorbed.
In addition, Osram have introduced a special feature; the EQUI (equipotential) connection. This is shown, as right, on the driver
This connection allows for different parts of the lighting system to be at the same potential, thus significantly reducing the overvoltages that could occur on the LED module.
In the case of an overvoltage pulse, the EQUI connection allows its discharge through the heatsink of the LED module or through the metal housing of the luminaire. Thus the surge current no longer flows through the LED module.
General guidance on protection from lightning strikes can be found in BS EN 62305-3:2011. There is also a very useful document on Surge Protection Devices produced by Beama. Of course, all electrical installations should conform to local Codes of Practice such as BS 7671.
A transient overvoltage or surge is a short duration increase in voltage measured between two or more conductors. In this context short means anything from microseconds to a few milliseconds in duration.
The increase in voltage will vary from a few volts to thousands of volts.
This voltage exists between two or more conductors. For a mains power supply, these conductors would be the line/phase, neutral and earth.
‘Transient overvoltage’, is technically and descriptively the best terminology. However, transients are also referred to as surges, spikes and glitches. The term ‘surge’, though widely used, should be used with caution. In some parts of the world, the UK amongst them, surge is used by the electricity supply industry to refer to a sustained overvoltage of several cycles duration.